1. Field of the Invention
The present invention relates to a multilayer structure in semiconductor material, especially for a solid state laser. More generally, the invention has applications in optoelectronics, and monolithic integration in opto- and microelectronics.
2. State of the Prior Art
Silicon Si and gallium arsenide GaAs are currently the most widely used semiconductor materials. Although a very high scale of integration has been obtained in microelectronics using silicon, optoelectronics has developed from heterostructure lasers with semiconductor materials from groups III and V of the Mendeleevian classification, such as GaAs/GaAlAs on GaAs substrate and such as GaInAs/AlInAs or GaInAs/InP on InP substrate. The integration on a common substrate of optoelectronic and microelectronic devices with complementary functions produced from different materials, e.g. material from groups III and V and silicon, is a particularly appealing perspective which has given rise to intensive research work in recent years.
A silicon substrate has numerous advantages: solidity, perfection, high thermal conductivity, low cost, etc. It is mainly the deposition of III-V compounds on silicon that has mainly been studied. Much progress has been achieved concerning this type of epitaxial growth and has enabled the obstacles encountered to be partially overcome: the difficulty in depositing a polar material such as III-V semiconductors on a non-polar material such as silicon, and the major lattice parameter difference between these, e.g. 4% for GaAs on silicon.
In particular, this mismatch implies the presence of a very high quantity of dislocations in the first tens of nanometers of the epitaxially deposited material. These dislocations can be due to the preparation of the state of the surface upon which the epitaxial growth is carried out, and/or to a degradation over time of the crystallographic quality of the epitaxial semiconductor. Whatever their origin, these dislocations beget local inhomgeneities and can develop.
For instance, when the crystalline lattice parameter of a layer is lower than that of a second layer, the first layer is subjected to tension and dislocations occur in the layer interface. If the first layer is the active layer of a laser component, the performances of the laser component are highly dependent on the number of dislocations present. These defects do, of course, affect the threshold current of this component with minority carriers, but also affect its ageing. When the component is operating, the presence of an electric field, and of high photon and carrier densities also help the displacement of existing defects and assist the generation of new defects.
For conventional optoelectronics applications, optoelectronic components are produced from III-V semiconductor heterostructures, such as GaAs/GaAlAs on GaAs substrate and such as GaInAs/AlInAs or GaInAs/InP on InP substrate. The dislocation rates obtained during the growth of these structures by conventional techniques, like molecular beam epitaxy and vapor phase epitaxy from organometallic compounds, are in the region of 10.sup.4 /cm.sup.2. The rates are compatible with proper functioning of these optoelectronic components, as witnessed by the large-scale commercial use of some of these, such as GaAs/GaAlAs laser diodes with 0.85-.mu.m wavelength; however, in certain cases, the displacement and multiplication of the dislocations during operating of the components have been observed and correlated with lifetime problems of these components. The dislocation-related problems, latent in this instance, become crucial if these defects are more numerous in the material used.
In the case of the growth of a III-V material on silicon, it is therefore primordial to seek as low a dislocation rate as possible in the structure. The dislocation rates in the superficial layer of the material remain in the region of 10.sup.6 to 10.sup.7 dislocations per cm.sup.2. The number of dislocations tends to decrease with the thickness of the deposit, but depositions cannot exceed 4 to 5 microns of material. Beyond that, the very major difference in coefficients of expansion between the silicon and the III-V material entails the formation of numerous cracks in the epitaxial layer when the growth temperature, in the region of 500.degree. to 600.degree. C., drops to room temperature. This failure of the attempts to reduce the number of dislocations is charged with consequences as regards the stability of laser components fabricated on silicon substrates.
To illustrate the results obtained, consideration is directed to the case of a GaAs structure on Si, which is the case that has been studied most intensely. No instance of room-temperature continuous-emission operation has been observed to date for double heterostructure lasers, as the component degrades below the laser threshold. On the other hand, a room-temperature operation in the pulsed mode, then in the continuous mode for short periods in the region of one minute has been observed for quantum well lasers, i.e., structures with confinement separated by index gradient. The reduction of the dimensions of the active layer is therefore clearly beneficial for the operating of the optoelectronic component. This result is related to several favourable factors. The use of the separate confinement concept brings about a reduction of the threshold current of the component, and therefore an increased stability of the latter. Moreover, there is a reduction in the size of the active layer which is the only region of the structure in which the two types of carriers are simultaneously present: the diffusion of the carriers before recombination then occurs in a plane and the dislocations no longer occur in the carrier capture except by their line fragment intersecting the active layer. However, even with these quantum well structures, the components obtained are not very stable, and have lifetime-related problems.